1. Field of the Invention
The present invention relates to a servo welding gun for a spot welding operation in which a welding tip (electrode) is driven by a servomotor to control a position of a distal end of the welding tip, and in particular to a method of and a device for setting a reference position of the movable welding lip of the servo welding gun which is attached to an industrial robot.
2. Description of the Related Art
In performing a spot welding operation by a servo welding gun attached to a robot, it is a common practice to control a position of a movable welding tip of the servo welding gun for applying a predetermined pressure to a workpiece as an object of welding between the movable welding tip and a stationary welding tip. For controlling the position of the movable welding tip in the spot welding, the movable welding tip is brought into contact with the stationary welding tip and a position of the movable welding tip in contact with the station welding tip is set and registered as a reference position (zero position) of an axis of the movable welding tip, before performing a teaching operation to the robot. The operation of setting the reference position has been performed manually by an operator for moving the movable welding tip to make contact with the stationary welding tip and setting the contact position as the reference position. The position of the movable welding tip is controlled on the basis of the set reference position in performing the spot welding operation. Specifically, after the welding gun is positioned with the distal end of the stationary welding tip in contact with one side of a workpiece, the movable tip is moved towards the stationary welding tip so that the distal end of the movable welding tip is positioned at a pressing position where a predetermined pressure is applied to the workpiece between the movable and stationary welding tips and an electric current is supplied therebetween.
The distal end of the welding tip is abraded to shorten the welding tip with a long time use, so that the predetermined pressure is not applied to a workpiece when the movable welding tip is moved to the pressing position to fail in performing an appropriate spot welding. Conventionally, the pressing position of the movable welding tip is compensated by an amount of abrasion of the welding tip. In order to determine the abrasion amount, an original position of the welding tip before the teaching operation (before abrasion of the welding tip) is stored in advance and the abrasion amount is determined as a difference between positions of the welding tip before and after abrasion of the welding tip.
FIGS. 7a-7e show conventional operations for determining the reference position (zero position) of the axis of the movable welding tip and the original position of the movable welding tip before a teaching operation (when the welding tip is not abraded), and an spot welding operation with compensation of the abrasion amount.
As shown in FIG. 7a, a movable welding tip 21 is moved so that a distal end of the movable welding tip 21 is brought into contact with a distal end of a stationary welding tip 22 through manual operation by an operator visually confirming the position of the distal end of the movable welding tip 21, and a position of the axis of the movable welding tip 21 deemed to be in contact with the stationary welding tip 22 is registered as a reference position (zero position). Further, in order to obtain a correction amount for compensating the abrasion of the movable welding tip 21, the servo gun 20 is moved to take predetermined position and posture where the movable welding tip 21 and the stationary welding tip 22 are positioned with a jig 30 fixed at a predetermined position therebetween, as shown in FIG. 7b, then the movable welding tip 22 is brought into contact with the jig 30 and the position of the movable welding tip 21 deemed to be in contact with the stationary welding tip 22 is registered as an original position of the movable welding tip 21 before abrasion. In the similar manner or other known method, an original position of the distal end of the stationary welding tip 22 before abrasion is stored.
As shown in FIG. 7c, in performing welding operations, the servo gun 20 is moved to a position where the distal end of the stationary welding tip 22 is brought into contact with one side of a workpiece 40 and then the movable. welding tip 21 is driven towards the stationary welding tip 22 to the pressing position where the predetermined pressure is applied to the workpiece 40 between the movable and stationary welding tips 21 and 22, and an electric current is supplied therebetween for spot welding the workpiece 40.
After a long time use, the welding tips 21 and 22 are abraded to fail in applying the predetermined pressure to the workpiece 40 therebetween, as schematically shown in FIG. 7d. In FIG. 7d, it should be noted that an abrasion amount of the stationary welding tip 22 is compensated and the servo gun 20 is positioned so that the stationary welding tip 22 is brought into contact with a lower surface of the workpiece 40. Thus, a distance “d” between the distal end of the movable welding tip 21 and the pressing position, which is depicted exaggerated, represents a sum of abraded amounts of the movable welding tip 21 and the stationary welding tip 22.
In this state, an appropriate welding is not performed. Before coming into such state, the servo welding gun 20 is moved to the predetermined position again and the movable welding tip 21 is brought into contact with the jig 30 and the position of the movable welding tip 21 in contact with the jig 30 is determined and a difference between the obtained position and the stored original position is determined as an amount of abrasion of the movable welding tip 21. The motion amount of the movable welding tip 21 is corrected based on the correction amount d which is a sum of the abrasion amount of the movable welding tip 21 and the stationary welding tip 22, so that the distal end of the movable welding tip 21 is moved to the pressing position. With this correction, the predetermined pressure is applied to the workpiece 40 by the movable and stationary welding tips 21 and 22, as shown in FIG. 7e. 
In the above described conventional method, a predetermined pressing position is corrected by abrasion amounts of the welding tips. In the practical use, a predetermined spot welding operation may be performed without correction of the pressing position in view of welding efficiency with being aware of deterioration of welding quality.
With the above conventional method, since the operation of obtaining the reference position of the welding tip of the servo welding gun 20 has to be carried out manually depending on an operator's visual confirmation, dispersion of precision of the reference position, the distal end of the movable welding tip does not reach the predetermined pressing position to fail in applying the predetermined pressure on the workpiece, to raise a problem in welding quantities. Further, it is laborious and takes a considerably long time to carry out the setting of the reference position to lower all operation efficiency of the welding operation. Furthermore, in the method of performing a spot welding operation while correcting the motion amount based on the correction amount d, it is necessary to determine and store the original position of the welding tip before abrasion in order to obtain the correction amount. This operation is required every time when the reference position is set, to lower the operation efficiency and raise a problem of maintenance.
In addition, a cycle time of spot welding operations is changed in the case of performing the welding operations with correction of abrasion amount and in the case of performing the welding operations without performing the correction of abrasion amount immediately after the teaching operation.
As stated, in the case of continuing the welding operation without performing the correction, the appropriate pressure is not obtained to rise a problem of lowering the welding quality.